U.S. patent number 9,490,418 [Application Number 14/092,077] was granted by the patent office on 2016-11-08 for acoustic resonator comprising collar and acoustic reflector with temperature compensating layer.
This patent grant is currently assigned to Avago Technologies General IP (Singapore) Pte. Ltd.. The grantee listed for this patent is Avago Technologies General IP (Singapore) Pte. Ltd.. Invention is credited to Dariusz Burak, John Choy, Kevin J. Grannen, Qiang Zou.
United States Patent |
9,490,418 |
Burak , et al. |
November 8, 2016 |
Acoustic resonator comprising collar and acoustic reflector with
temperature compensating layer
Abstract
An acoustic resonator structure includes an acoustic reflector
over a cavity formed in a substrate, the acoustic reflector
including a layer of low acoustic impedance material stacked on a
layer of high acoustic impedance material. The acoustic resonator
further includes a bottom electrode on the layer of low acoustic
impedance material, a piezoelectric layer on the bottom electrode,
a top electrode on the piezoelectric layer, and a collar formed
outside a main membrane region defined by an overlap between the
top electrode, the piezoelectric layer and the bottom electrode.
The collar has an inner edge substantially aligned with a boundary
of or overlapping the main membrane region. The layer of the low
acoustic impedance material includes a temperature compensating
material having a positive temperature coefficient for offsetting
at least a portion of a negative temperature coefficient of the
piezoelectric layer, the bottom electrode and the top
electrode.
Inventors: |
Burak; Dariusz (Fort Collins,
CO), Choy; John (Westminster, CO), Grannen; Kevin J.
(Thornton, CO), Zou; Qiang (Fort Collins, CO) |
Applicant: |
Name |
City |
State |
Country |
Type |
Avago Technologies General IP (Singapore) Pte. Ltd. |
Singapore |
N/A |
SG |
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Assignee: |
Avago Technologies General IP
(Singapore) Pte. Ltd. (Singapore, SG)
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Family
ID: |
50880192 |
Appl.
No.: |
14/092,077 |
Filed: |
November 27, 2013 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20140159548 A1 |
Jun 12, 2014 |
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Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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13767754 |
Feb 14, 2013 |
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13955774 |
Jul 31, 2013 |
9246473 |
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13781491 |
Feb 28, 2013 |
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13663449 |
Oct 29, 2012 |
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13208883 |
Aug 12, 2011 |
9083302 |
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13074262 |
Mar 29, 2011 |
9136818 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01L
41/053 (20130101); H03H 9/173 (20130101); H03H
9/02118 (20130101) |
Current International
Class: |
H03H
9/15 (20060101); H03H 9/02 (20060101); H03H
9/17 (20060101); H01L 41/053 (20060101) |
Field of
Search: |
;310/311-371
;333/186-197 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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Apr 2008 |
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CN |
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0880227 |
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Aug 2012 |
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EP |
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10173479 |
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Jun 1998 |
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JP |
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Jan 2003 |
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JP |
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Jul 2006 |
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JP |
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2007-208845 |
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Aug 2007 |
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JP |
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Sep 2008 |
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JP |
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Sep 2008 |
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JP |
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Jun 2010 |
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JP |
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Jun 2002 |
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KR |
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1020030048917 |
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Jun 2003 |
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KR |
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Jul 1999 |
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WO |
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May 2005 |
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WO |
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2006079353 |
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Aug 2006 |
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WO |
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WO-2007085332 |
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Aug 2007 |
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WO |
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2013065488 |
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May 2013 |
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WO |
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Primary Examiner: Dougherty; Thomas
Parent Case Text
CROSS REFERENCE TO RELATED APPLICATIONS
This application is a continuation-in-part under 37 C.F.R.
.sctn.1.53(b) of commonly owned U.S. patent application Ser. No.
13/767,754 entitled "Acoustic Resonator Having Temperature
Compensation," filed on Feb. 14, 2013, which is hereby incorporated
by reference in its entirety. This application is also a
continuation-in-part under 37 C.F.R. .sctn.1.53(b) of commonly
owned U.S. patent application Ser. No. 13/955,774 entitled
"Acoustic Resonator Comprising Collar, Frame and Perimeter
Distributed Bragg Reflector," filed on Jul. 31, 2013, which is a
continuation-in-part of commonly owned U.S. patent application Ser.
No. 13/781,491 entitled "Acoustic Resonator Having Collar and
Frame," filed on Feb. 28, 2013, which is a continuation-in-part of
commonly owned U.S. patent application Ser. No. 13/663,449 entitled
"Acoustic Resonator Having Collar Structure," filed on Oct. 29,
2012, which are hereby incorporated by reference in their
entireties. U.S. patent application Ser. No. 13/955,774 is also a
continuation-in-part under 37 C.F.R. .sctn.1.53(b) of commonly
owned U.S. patent application Ser. No. 13/208,883 entitled "Stacked
Bulk Acoustic Resonator Comprising a Bridge and an Acoustic
Reflector along a Perimeter of the Resonator," filed on Aug. 12,
2011 (published as U.S. Patent App. Pub. No. 2012/0218059), which
is a continuation-in-part application of commonly owned U.S. patent
application Ser. No. 13/074,262 entitled "Stacked Acoustic
Resonator Comprising Bridge," filed on Mar. 29, 2011 (published as
U.S. Patent App. Pub. No. 2012/0218055), which are hereby
incorporated by reference in their entireties.
Claims
The invention claimed is:
1. An acoustic resonator, comprising: an acoustic reflector
disposed on a substrate over a cavity formed in the substrate, the
acoustic reflector comprising a layer of low acoustic impedance
material stacked on a layer of high acoustic impedance material; a
bottom electrode disposed on the layer of low acoustic impedance
material of the acoustic reflector; a piezoelectric layer disposed
on the bottom electrode; a top electrode disposed on the
piezoelectric layer; and a collar formed outside a main membrane
region defined by an overlap between the top electrode, the
piezoelectric layer and the bottom electrode, the collar having an
inner edge substantially aligned with a boundary of or overlapping
the main membrane region, wherein the layer of the low acoustic
impedance material comprises a temperature compensating material
having a positive temperature coefficient for offsetting at least a
portion of a negative temperature coefficient of the piezoelectric
layer, the bottom electrode and the top electrode.
2. The acoustic resonator of claim 1, wherein the low acoustic
impedance material comprises borosilicate glass (BSG) or
tetra-ethyl-ortho-silicate (TEOS).
3. The acoustic resonator of claim 1, wherein the collar defines a
collar region having a cutoff frequency that is substantially the
same as a cutoff frequency of the main membrane region.
4. The acoustic resonator of claim 3, wherein the collar is formed
on a top surface of the top electrode and a planarization layer
adjacent the top electrode.
5. An acoustic resonator, comprising: an acoustic reflector
disposed over a cavity formed in a substrate, the acoustic
reflector comprising a layer of low acoustic impedance material
stacked on a layer of high acoustic impedance material; a bottom
electrode disposed on the layer of low acoustic impedance material
of the acoustic reflector; a piezoelectric layer disposed on the
bottom electrode; a top electrode disposed on the piezoelectric
layer; and a collar formed outside a main membrane region defined
by an overlap between the top electrode, the piezoelectric layer
and the bottom electrode, the collar having an inner edge
substantially aligned with a boundary of or overlapping the main
membrane region, wherein the layer of the low acoustic impedance
material comprises a temperature compensating material having a
positive temperature coefficient for offsetting at least a portion
of a negative temperature coefficient of the piezoelectric layer,
the bottom electrode and the top electrode, and wherein the collar
is formed between the bottom electrode and the piezoelectric
layer.
6. The acoustic resonator of claim 1, further comprising: at least
one frame disposed within the main membrane region and having an
outer edge substantially aligned with the boundary of the main
membrane region.
7. The acoustic resonator of claim 6, wherein the at least one
frame comprises an add-on frame.
8. The acoustic resonator of claim 6, wherein the at least one
frame comprises a composite frame.
9. The acoustic resonator of claim 6, wherein the at least one
frame comprises a frame disposed at a bottom portion of the top
electrode.
10. The acoustic resonator of claim 9, wherein the at least one
frame comprises another frame disposed at one of a top portion or a
bottom portion of the bottom electrode.
11. The acoustic resonator of claim 6, wherein the at least one
frame comprises a frame disposed at a bottom portion of the bottom
electrode.
12. The acoustic resonator of claim 1, wherein the collar comprises
borosilicate glass, carbon-doped silicon oxide, silicon carbide,
silicon nitride, aluminum oxide, aluminum nitride, zinc oxide, lead
zirconium titanate, diamond or diamond-like carbon.
13. The acoustic resonator of claim 6, wherein the frame comprises
a layer of copper, molybdenum, aluminum, tungsten, iridium,
borosilicate glass, carbon-doped silicon oxide, silicon carbide,
silicon nitride, aluminum oxide, aluminum nitride, zinc oxide, lead
zirconium titanate, diamond or diamond-like carbon.
14. The acoustic resonator of claim 1, wherein the piezoelectric
layer is doped with at least one rare earth element for offsetting
at least a portion of degradation of an electromechanical coupling
coefficient of the acoustic resonator caused by the temperature
compensating material.
15. A thin-film bulk acoustic resonator (FBAR), comprising: a
substrate defining a cavity; a distributed Bragg reflector (DBR)
disposed on a top surface of the substrate over the cavity, the DBR
comprising at least one layer of low acoustic impedance material
having a positive temperature coefficient; an acoustic stack
arranged on the DBR over the cavity, the acoustic stack comprising
a piezoelectric layer sandwiched between bottom and top electrode
layers, and having a main membrane region defined by an overlap
between the bottom electrode, the piezoelectric layer, and the top
electrode; and a collar arranged outside the main membrane region,
the collar defining a collar region having a cutoff frequency that
is substantially the same as a cutoff frequency of the main
membrane region, wherein the positive temperature coefficient of
the at least one layer of low acoustic impedance material offsets
at least a portion of a negative temperature coefficient of the
piezoelectric layer, the bottom electrode layer and the top
electrode layer.
16. The FBAR of claim 15, further comprising: a frame formed in or
on at least one of the bottom and top electrodes, and disposed
within the main membrane region.
17. The FBAR of claim 16, wherein the collar is formed on a surface
of the bottom electrode.
18. The FBAR of claim 16, wherein the collar is formed on a surface
of the top electrode and a planarization layer adjacent to the top
electrode.
19. The acoustic resonator of claim 1, wherein the piezoelectric
layer is doped with at least one rare earth element for offsetting
at least a portion of degradation of an electromechanical coupling
coefficient of the acoustic resonator caused by the low acoustic
impedance material having the positive temperature coefficient.
20. The acoustic resonator of claim 5, wherein the collar defines a
collar region having a cutoff frequency that is substantially the
same as a cutoff frequency of the main membrane region.
Description
BACKGROUND
Acoustic resonators can be used to implement signal processing
functions in various electronic applications. For example, some
cellular phones and other communication devices use acoustic
resonators to implement frequency filters for transmitted and/or
received signals. Several different types of acoustic resonators
can be used according to different applications, with examples
including bulk acoustic wave (BAW) resonators such as thin film
bulk acoustic resonators (FBARs), coupled resonator filters (CRFs),
stacked bulk acoustic resonators (SBARs), double bulk acoustic
resonators (DBARs), and solidly mounted resonators (SMRs). An FBAR,
for example, includes a piezoelectric layer between a first
(bottom) electrode and a second (top) electrode over a cavity. BAW
resonators may be used in a wide variety of electronic
applications, such as cellular telephones, personal digital
assistants (PDAs), electronic gaming devices, laptop computers and
other portable communications devices. For example, FBARs may be
used for electrical filters and voltage transformers.
An acoustic resonator typically comprises a layer of piezoelectric
material sandwiched between two plate electrodes in a structure
referred to as an acoustic stack. Where an input electrical signal
is applied between the electrodes, reciprocal or inverse
piezoelectric effect causes the acoustic stack to mechanically
expand or contract depending on the polarization of the
piezoelectric material. As the input electrical signal varies over
time, expansion and contraction of the acoustic stack produces
acoustic waves that propagate through the acoustic resonator in
various directions and are converted into an output electrical
signal by the piezoelectric effect. Some of the acoustic waves
achieve resonance across the acoustic stack, with the resonance
frequency being determined by factors such as the materials,
dimensions, and operating conditions of the acoustic stack. These
and other mechanical characteristics of the acoustic resonator
determine its frequency response.
In general, an acoustic resonator comprises different lateral
regions that may be subject to different types of resonances, or
resonance modes. These lateral regions can be characterized, very
broadly, as a main membrane region and peripheral regions, where
the main membrane region is defined, roughly, by an overlap between
the two plate electrodes and the piezoelectric material, and the
peripheral regions are defined as areas outside the main membrane
region. Two peripheral regions, in particular, are defined as a
region located between the edge of the main membrane region and
edge of the air-cavity, and a region of an overlap of at least one
plate electrode and the piezoelectric material with the substrate.
The main membrane region is subject to electrically excited modes
generated by the electric field between the two plate electrodes,
and both the main membrane and the peripheral regions are subject
to certain derivative modes generated by scattering of energy in
the electrically excited modes. The electrically excited modes
comprise, for instance, a piston mode formed by longitudinal
acoustic waves with boundaries at the edges of the main membrane
region. The derivative modes comprise, for instance, lateral modes
formed by lateral acoustic waves excited at the edges of the main
membrane region and the peripheral regions.
The lateral modes facilitate continuity of appropriate mechanical
particle velocities and stresses between the electrically driven
main membrane region and the essentially non-driven peripheral
regions. They can either propagate freely (so called propagating
modes) or exponentially decay (so called evanescent and complex
modes) from the point of excitation. They can be excited both by
lateral structural discontinuities (e.g., an interface between
regions of different thicknesses in the main membrane region, or an
edge of a top or bottom electrode) or by electric field
discontinuities (e.g., an edge of a top electrode where the
electric field is terminated abruptly).
The lateral modes generally have a deleterious impact on the
performance of an acoustic resonator. Accordingly, some acoustic
resonators include ancillary structural features designed to
suppress, inhibit, or mitigate the lateral modes. For example, a
collar may be formed by a dielectric material outside the boundary
of the main membrane region to allow a smooth decay of evanescent
modes emanating from the boundary and improve confinement of
mechanical motion to the main membrane region. In another example,
a frame may be formed by a conductive or dielectric material within
the boundary of the main membrane region to minimize scattering of
electrically excited piston mode at top electrode edges and improve
confinement of mechanical motion to the main membrane region.
The conventional implementation of these ancillary structural
features has a number of potential shortcomings. For instance,
depending on their specific design, they may be a source of
additional scattering of the piston mode which may outweigh their
benefits. Additionally, they may require the presence of certain
additional materials that can deleteriously redistribute the
acoustic energy in the acoustic stack, such as relatively soft
planarization layers. Also, some design choices may produce only
modest performance improvements while significantly driving up
cost. Moreover, the formation of ancillary structural features may
degrade structural stability or interfere with the formation of
overlying layers.
In addition, conventional FBARs rely on strong confinement of
electrically excited piston mode. Strong confinement is provided by
the edges of the top and bottom electrodes, as well as ancillary
structural features, such as air-bridges and conventional outside
frames. While the apparent advantage of strong confinement is that
it prevents strong electrical excitation of mechanical motion at
the edge of the top electrode, it also provides significant
acoustic discontinuities, leading to scattering of energy out of
the desired piston mode into undesired extensional, shear, flexural
and dilatational modes of the whole structure. Accordingly, in view
of these and other shortcomings of conventional acoustic resonator
structures, there is a general need for improved acoustic resonator
designs.
BRIEF DESCRIPTION OF THE DRAWINGS
The illustrative embodiments are best understood from the following
detailed description when read with the accompanying drawing
figures. It is emphasized that the various features are not
necessarily drawn to scale. In fact, the dimensions may be
arbitrarily increased or decreased for clarity of discussion.
Wherever applicable and practical, like reference numerals refer to
like elements.
FIG. 1A is a top view of an acoustic resonator according to a
representative embodiment.
FIG. 1B is a cross-sectional view of an acoustic resonator
according to a representative embodiment.
FIG. 1C is a cross-sectional view of an acoustic resonator
according to another representative embodiment.
FIG. 1D is a cross-sectional view of an acoustic resonator
according to another representative embodiment.
FIG. 1E is a cross-sectional view of an acoustic resonator
according to another representative embodiment.
FIG. 1F is a cross-sectional view of an acoustic resonator
according to another representative embodiment.
FIG. 2A is a cross-sectional view of an acoustic resonator
according to another representative embodiment.
FIG. 2B is a cross-sectional view of an acoustic resonator
according to another representative embodiment.
FIG. 2C is a cross-sectional view of an acoustic resonator
according to another representative embodiment.
FIG. 2D is a cross-sectional view of an acoustic resonator
according to another representative embodiment.
FIG. 3A is a cross-sectional view of an acoustic resonator,
excluding frames, according to another representative
embodiment.
FIG. 3B is a cross-sectional view of an acoustic resonator,
excluding frames, according to another representative
embodiment.
FIG. 3C is a cross-sectional view of an acoustic resonator,
excluding collars, according to another representative
embodiment.
FIG. 3D is a cross-sectional view of an acoustic resonator,
excluding collars, according to another representative
embodiment.
FIG. 4 is a graph illustrating the quality factor (Q-factor) and
parallel resistance Rp of the acoustic resonators of FIGS. 3A and
1B, as compared to an acoustic resonator having only a temperature
compensating acoustic impedance layer.
DETAILED DESCRIPTION
In the following detailed description, for purposes of explanation
and not limitation, example embodiments disclosing specific details
are set forth in order to provide a thorough understanding of the
present teachings. However, it will be apparent to one having
ordinary skill in the art having the benefit of the present
disclosure that other embodiments according to the present
teachings that depart from the specific details disclosed herein
remain within the scope of the appended claims. Moreover,
descriptions of well-known apparatuses and methods may be omitted
so as to not obscure the description of the example embodiments.
Such methods and apparatuses are clearly within the scope of the
present teachings.
The terminology used herein is for purposes of describing
particular embodiments only, and is not intended to be limiting.
The defined terms are in addition to the technical, scientific, or
ordinary meanings of the defined terms as commonly understood and
accepted in the relevant context.
The terms "a", "an" and "the" include both singular and plural
referents, unless the context clearly dictates otherwise. Thus, for
example, "a device" includes one device and plural devices. The
terms "substantial" or "substantially" mean to within acceptable
limits or degree. The term "approximately" means to within an
acceptable limit or amount to one of ordinary skill in the art.
Relative terms, such as "above," "below," "top," "bottom," "upper"
and "lower" may be used to describe the various elements'
relationships to one another, as illustrated in the accompanying
drawings. These relative terms are intended to encompass different
orientations of the device and/or elements in addition to the
orientation depicted in the drawings. For example, if the device
were inverted with respect to the view in the drawings, an element
described as "above" another element, for example, would now be
below that element. Where a first device is said to be connected or
coupled to a second device, this encompasses examples where one or
more intermediate devices may be employed to connect the two
devices to each other. In contrast, where a first device is said to
be directly connected or directly coupled to a second device, this
encompasses examples where the two devices are connected together
without any intervening devices other than electrical connectors
(e.g., wires, bonding materials, etc.).
The present teachings relate generally to acoustic resonators such
as film bulk acoustic wave resonators (FBARs) or solidly mounted
resonators (SMRs), although the discussion is directed to FBARs for
the sake of convenience. Certain details of acoustic resonators,
including materials and methods of fabrication, may be found in one
or more of the following commonly owned U.S. Patents and Patent
Applications: U.S. Pat. No. 6,107,721 to Lakin; U.S. Pat. Nos.
5,587,620, 5,873,153, 6,507,983, 6,384,697, 7,275,292 and 7,629,865
to Ruby et al.; U.S. Pat. No. 7,280,007 to Feng, et al.; U.S.
Patent App. Pub. No. 2007/0205850 to Jamneala et al.; U.S. Pat. No.
7,388,454 to Ruby et al.; U.S. Patent App. Pub. No. 2010/0327697 to
Choy et al.; U.S. Patent App. Pub. No. 2010/0327994 to Choy et al.,
U.S. patent application Ser. No. 13/658,024 to Nikkel et al.; U.S.
patent application Ser. No. 13/663,449 to Burak et al.; U.S. patent
application Ser. No. 13/660,941 to Burak et al.; U.S. patent
application Ser. No. 13/654,718 to Burak et al.; U.S. Patent App.
Pub. No. 2008/0258842 to Ruby et al.; and U.S. Pat. No. 6,548,943
to Kaitila et al. The disclosures of these patents and patent
applications are hereby specifically incorporated by reference in
their entireties. It is emphasized that the components, materials
and method of fabrication described in these patents and patent
applications are representative and other methods of fabrication
and materials within the purview of one of ordinary skill in the
art are contemplated.
In certain embodiments described below, an acoustic resonator
comprises a piezoelectric layer disposed between top and bottom
electrodes, and an acoustic reflector, such as a distributed Bragg
reflector (DBR), disposed over a cavity defined by the substrate.
The acoustic reflector may be formed of a single layer or multiple
layers, where at least one of the layers comprises a temperature
compensating layer, e.g., formed of material having a temperature
coefficient that offsets temperature coefficient(s) of other
component(s) of the acoustic resonator. The acoustic reflector may
be formed of one or more pairs of layers formed under a bottom
electrode where it minimizes a detrimental impact of a so-called
"dead-FBAR" region in which acoustic vibrations of the acoustic
resonator may be attenuated through mechanical scattering of the
electrically excited motion at a boundary between the bottom
electrode and an underlying substrate and through the transducer
effect in the region where FBAR acoustic stack overlaps the
substrate. Each pair of layers consists of a low acoustic impedance
layer stacked on a high acoustic impedance layer.
The acoustic resonator may further include a collar disposed
outside a main membrane region and/or a frame disposed within the
main membrane region. The collar typically has an inner edge
substantially aligned with a boundary of the main membrane region
or somewhat overlapping the main membrane region, and the frame
typically has an outer edge substantially aligned with the boundary
of the main membrane region. The cavity may be disposed in the
substrate within the main membrane region. Generally, a collar
couples the evanescent thickness extensional (eTE1) and piston
modes of a main membrane region to the evanescent thickness
extensional mode of a collar region, and a frame suppresses
excitation of propagating modes.
The collar may be formed of a relatively thick dielectric region of
finite width, and may be located in various alternative locations,
such as above the top electrode, below the bottom electrode, or
between the bottom electrode and the piezoelectric layer. The
collar may also be divided into multiple layers and formed in more
than one of the above locations. Also, the collar may be formed
inside other features of the acoustic resonator, for instance,
inside the piezoelectric layer. A region of the acoustic resonator
above and below the collar will be referred to as a collar
region.
The collar is typically designed so that its cutoff frequency is
substantially the same as the cutoff frequency in the main membrane
region, and its main non-propagating mode (evanescent mode, for
instance) has substantially the same modal distribution as the
piston mode in the main membrane region. This prevents acoustic
energy in the piston mode from being converted into unwanted
propagating modes in the collar region and propagating and
evanescent modes in the main membrane region. If excited,
propagating modes in the collar region in general may lead to
energy loss due to acoustic radiation to the region outside of
acoustic resonator. Similarly, if excited, propagating and
evanescent modes inside the main membrane region may in general
produce lateral voltage gradients, which may lead to lateral
current flows and energy loss due to the Joule heating. Thus, the
collar may improve confinement of the piston mode within the main
membrane region while suppressing the excitation of unwanted
spurious lateral modes inside and outside of the main membrane
region. This, in turn, may reduce overall acoustic scattering loss
and enhance the parallel resistance Rp and the quality factor
(Q-factor) of the acoustic resonator.
In the absence of the collar, there may be a significant acoustic
impedance discontinuity at the edge of the top electrode for an
electrically excited piston mode. Because the electric field is
also terminated at the edge of top electrode, that edge will cause
both mechanical and electrical excitation of evanescent,
propagating and complex modes supported by the structures both
inside and outside of the main membrane region. Evanescent and
complex modes decay exponentially, so a wide enough collar
structure will suppress them. Moreover, propagating modes may be
suppressed by forming the collar structure with a proper width.
Additionally, a collar structure extending over (or under) the top
electrode may act as an integrated frame, thus it may minimize the
amplitude of electrically excited piston mode before the top
electrode edge and provide additional acoustic impedance
discontinuities to suppress propagating modes. Thus, in the
presence of a properly designed collar, most of the piston mode
energy at the top electrode edge may couple to the evanescent mode
in the collar region, which may then decay exponentially and become
efficiently suppressed inside a wide enough collar structure. When
the collar overlaps with the substrate, the acoustic reflector also
prevents the evanescent and complex modes supported by the collar
from coupling to the substrate.
The frame is formed by adding a layer of material, usually an
electrically conducting material (although dielectric material is
possible as well), to the top and/or bottom electrode. The frame
can be either a composite frame or an add-on frame, for example. A
composite frame has integrated lateral features, formed of aluminum
(Al) and molybdenum (Mo), for example, and is formed by embedding
material within the top or bottom electrode, typically with an
exposed upper or lower surface being coplanar with an upper or
lower surface of the top or bottom electrode. An add-on frame is
formed by depositing the material above or below of a layer forming
either the bottom or top electrode along a perimeter of the main
membrane region. The use of a composite frame can simplify
fabrication of the acoustic resonator with regard to application of
layers on planar surfaces. For instance, it can prevent the
formation of outcroppings in overlying layers, which can preserve
the structural stability of the acoustic resonator. A region of the
acoustic resonator above and below the frame will be collectively
referred to as a frame region.
The frame generally suppresses electrically excited piston mode in
the frame region, and it reflects and otherwise resonantly
suppresses propagating eigenmodes in lateral directions, with both
effects simultaneously improving operation of the acoustic
resonator. This is because the frame's presence generally produces
at least one of a cutoff frequency mismatch and an acoustic
impedance mismatch between the frame region and other portions of
the main membrane region. A frame that lowers the cutoff frequency
in the frame region as compared to the main membrane region will be
referred to as a Low Velocity Frame (LVF), while a frame that
increases the cutoff frequency in the frame region as compared to
the main membrane region will be referred to as a High Velocity
Frame (HVF). The reasoning behind this nomenclature is that for
composite frames (for which thicknesses of the frame and main
membrane regions are substantially the same), an increase or
decrease of the cutoff frequency is substantially equivalent to an
increase or decrease an effective sound velocity of the acoustic
stack forming the frame, respectively.
A composite or add-on frame with lower effective sound velocity
than the corresponding effective sound velocity of a main membrane
(i.e., an LVF) generally increases parallel resistance Rp and
Q-factor of the acoustic resonator above the cutoff frequency of
the main membrane region. Conversely, a composite or add-on frame
with a higher effective sound velocity than the corresponding
effective sound velocity of a main membrane (i.e., an HVF)
generally decreases series resistance Rs and increases Q-factor of
the acoustic resonator below the cutoff frequency of the main
membrane region. A typical low velocity frame, for example,
effectively provides a region with significantly lower cutoff
frequency than the main membrane region and therefore minimizes the
amplitude of the electrically excited piston mode towards the edge
of the top electrode in the frame region. Furthermore, it provides
two interfaces (impedance miss-match planes), which increase
reflection of propagating eigenmodes. These propagating eigenmodes
are mechanically excited at membrane/frame interface, and both
mechanically and electrically excited at the top electrode edge.
Where the width of the frame is properly designed for a given
eigenmode, it results in resonantly enhanced suppression of that
particular eigenmode. In addition, a sufficiently wide low velocity
frame provides a region for smooth decay of the evanescent and
complex modes, which are excited by similar mechanisms as the
propagating eigenmodes. The combination of the above effects yields
better energy confinement and higher Q-factor at a parallel
resonance frequency Fp.
Various additional examples of collars and frames, as well as
related materials and operating characteristics, are described in
the above cited U.S. patent application Ser. Nos. 13/663,449 and
13/660,941 to Burak et al., which are hereby incorporated by
reference in their entireties. As explained in those applications,
collars and frames can be placed in various alternative locations
and configurations relative to other portions of an acoustic
resonator, such as the electrodes and piezoelectric layer of an
acoustic stack. Additionally, their dimensions, materials, relative
positioning, and so on, can be adjusted to achieve specific design
objectives, such as a target resonance frequency, series resistance
Rs, parallel resistance Rp, or electromechanical coupling
coefficient Kt.sup.2. Although the following description presents
several embodiments in the form of FBAR devices, several of the
described concepts could be implemented in other forms of acoustic
resonators, such as SMRs, for example.
FIG. 1A is a top view of an acoustic resonator 100A according to a
representative embodiment, and FIGS. 1B-1F are cross-sectional
views of acoustic resonator 100A, taken along a line A-A' according
to different embodiments. The cross-sectional views correspond to
different variations of acoustic resonator 100A and will be
referred to, respectively, as acoustic resonators 100B-100F.
Acoustic resonators 100B-100F have many of the same features, so a
repetitive description of these features may be omitted in an
effort to avoid redundancy.
Referring to FIG. 1A, acoustic resonator 100A comprises a top
electrode 135 having five (5) sides, with a connection side 101
configured to provide an electrical connection to interconnect 102.
Interconnect 102 provides electrical signals to top electrode 135
to excite desired acoustic waves in a piezoelectric layer (not
shown in FIG. 1A) of acoustic resonator 100A.
FIGS. 1B-1F are cross-sectional diagrams illustrating acoustic
resonators, according to representative embodiments. In the
examples depicted in FIGS. 1B-1F (as well as the examples depicted
in FIGS. 2A to 4, discussed below), the acoustic resonator is an
FBAR, for convenience of explanation. However, it is understood
that other types of acoustic resonators may be included, without
departing from the scope of the present teachings. Each of the
acoustic resonators shown in FIGS. 1B to 1F includes an acoustic
reflector or acoustic mirror, such as a distributed Bragg reflector
(DBR), formed beneath the acoustic stack over the substrate and a
cavity formed in the substrate, as well as a temperature
compensating feature (e.g., temperature compensating layer) located
within the acoustic reflector. For example, one or more of the
acoustic impedance layers in the acoustic mirror may be formed of a
material enabling it to also serve as a temperature compensating
layer, e.g., having a positive temperature coefficient configured
to offset negative temperature coefficients of other materials in
the acoustic stack. It is understood that the same general
configurations may be included in acoustic resonators having frames
and/or collars in various locations, without departing from the
scope of the present teachings.
Referring to FIG. 1B, acoustic resonator 100B, which may be an
FBAR, for example, comprises a substrate 105, a cavity 110 (e.g.,
air cavity) and an acoustic reflector, indicated by illustrative
distributed Bragg reflector (DBR) 106, formed over a top surface of
the substrate 105 and the cavity 110. The DBR 106 includes one or
more pairs of acoustic impedance layers, indicated by
representative first acoustic impedance layer 107 and second
acoustic impedance layer 108, discussed below, and a temperature
compensating feature. A bottom (first) electrode 115 is disposed on
the DBR 106, and a first planarization layer 120 is disposed on the
DBR 106 adjacent to the bottom electrode 115. A piezoelectric layer
125 is disposed on the bottom electrode 115 and the first
planarization layer 120. A top (second) electrode 135 is disposed
on the piezoelectric layer 125. Collectively, the bottom electrode
115, the piezoelectric layer 125, and the top electrode 135
constitute an acoustic stack of acoustic resonator 100B. A second
planarization layer 130 is disposed on the piezoelectric layer 125
adjacent to the top electrode 135 to accommodate collar 140,
although the second planarization layer 130 is not needed if there
is no collar or if the collar is located elsewhere in the acoustic
stack, as discussed below.
The first and second acoustic impedance layers 107 and 108 may be
formed with respective thicknesses corresponding to a quarter
wavelength of a natural resonance frequency of acoustic resonator
100B, for example. Generally, the amount of acoustic isolation
provided by DBR 106 depends on the contrast between the acoustic
impedances of adjacent acoustic impedance layers, with a greater
amount of contrast creating better acoustic isolation. In some
embodiments, the DBR 106 is formed in pairs of dielectric materials
having contrasting acoustic impedances. For example, in the
depicted embodiment, the DBR 106 comprises a single pair of
acoustic impedance layers, first acoustic impedance layer 107 and
second acoustic impedance layer 108, where the first acoustic
impedance layer 107 is formed of a material having relatively low
acoustic impedance (which may be a relatively soft material) and
the second acoustic impedance layer 108 paired with the first
acoustic impedance layer 107 is formed of a material having
relatively high acoustic impedance (which may be a relatively hard
material).
Of course, in various embodiments, the DBR 106 may include other
numbers of acoustic impedance layers and/or acoustic impedance
layer pairs, e.g., to achieve specific design objectives, without
departing from the scope of the present teachings. For example, in
the presence of the cavity 110, the DBR 106 may be formed of a
single half-wavelength layer rather than a quarter-wavelength layer
in order to preserve the designed resonance frequency of acoustic
resonator 100B. In another example, the DBR 106 may be formed of
multiple pairs of acoustic impedance layers. When there are
additional acoustic impedance layer pairs in the DBR 106, each pair
likewise includes a first acoustic impedance layer (107) formed of
low acoustic impedance material stacked on a second acoustic
impedance layer (108) formed of high acoustic impedance material.
Various illustrative fabrication techniques of acoustic mirrors are
described by in U.S. Pat. No. 7,358,831 (Apr. 15, 2008), to Larson
III, et al., which is hereby incorporated by reference in its
entirety.
As mentioned above, the DBR 106 includes a temperature compensating
feature. That is, at least one of the acoustic impedance layers of
the DBR 106 having relatively low acoustic impedance (e.g., first
acoustic impedance layer 107) is formed of a material that also
provides temperature compensation for the acoustic resonator 100B.
Such materials include boron silicate glass (BSG),
tetra-ethyl-ortho-silicate (TEOS), silicon dioxide (SiO.sub.2), and
niobium molybdenum (NbMo), for example, which have positive
temperature coefficients. The positive temperature coefficient of
the temperature compensating acoustic impedance layer offsets
negative temperature coefficients of other materials in the
acoustic stack, including the piezoelectric layer 125, the bottom
electrode 115, and the top electrode 135, for example. Various
illustrative fabrication techniques of temperature compensating
layers are described by U.S. patent application Ser. No. 13/766,993
(filed Feb. 14, 2013), to Burak et al., which is hereby
incorporated by reference in its entirety.
The second acoustic impedance layer 108 paired with the first
acoustic impedance layer 107 is formed of a material having
relatively high acoustic impedance, such as tungsten (W) or
molybdenum (Mo), for example. In embodiments in which the DBR 106
includes more than one pair of first (low) and second (high)
acoustic impedance layers, the low acoustic impedance layer in each
pair may be formed of the same temperature compensating material as
the first acoustic impedance layer 107, a different temperature
compensating material, or a low acoustic impedance material having
no effective temperature compensating attributes, without departing
from the scope of the present teachings. For example, other low
acoustic impedance layers may be formed of materials, such as
borosilicate glass (BSG), tetra-ethyl-ortho-silicate (TEOS),
silicon oxide (SiO.sub.x) or silicon nitride (SiN.sub.x), where x
is an integer, carbon-doped silicon oxide (CDO), chemical vapor
deposition silicon carbide (CVD SiC), or plasma enhanced CVD SiC
(PECVD SiC). In another example, the additional low acoustic
impedance layers (odd acoustic impedance layers) may be formed of
carbon-doped silicon oxide (CDO), while the corresponding high
acoustic impedance layers (even acoustic impedance layers) paired
with low acoustic impedance layers may be formed of silicon nitride
(SiN.sub.x), where x is an integer. A benefit of this latter
pairing of materials is that the layer may be grown in a single
machine by depositing CDO onto a silicon wafer, for example, within
a first chamber, moving the wafer to a second chamber, depositing
silicon nitride on the wafer in the second chamber, moving the
wafer back into the first chamber, and so on. Of course, the low
and high acoustic impedance materials forming the stacked layers of
the DBR 106 may vary without departing from the scope of the
present teachings.
The relative thicknesses of the temperature compensating acoustic
impedance layer (first acoustic impedance layer 107) and the
non-temperature compensating acoustic impedance layer (second
acoustic impedance layer 108) should be optimized in order to
maximize the coupling coefficient for an allowable linear
temperature coefficient. This optimization may be accomplished, for
example, by modeling an equivalent circuit of the acoustic stack
using a Mason model, as would be apparent to one of ordinary skill
in the art. Although there is some degradation in the offsetting
effects of the temperature coefficient by making the first acoustic
impedance layer 107 thinner, the coupling coefficient of the
acoustic resonator 100B may be improved. An algorithm may be
developed to optimize the thicknesses of the first acoustic
impedance layer 107 and the second acoustic impedance layer 108 in
the DBR 106 in light of the trade-off between the temperature
coefficient and the coupling coefficient, for example, using a
multivariate optimization technique, such as a Simplex method, as
would be apparent to one of ordinary skill in the art. Such
optimization and corresponding considerations regarding temperature
compensation are also applicable to the other FBARs discussed
herein (e.g., acoustic resonators 100C to 300D, discussed
below).
Generally, the DBR 106 substantially eliminates "dead-FBAR" by
providing acoustic isolation of a connecting edge of the top
electrode 135 from the substrate 105. The DBR 106 also prevents
evanescent and complex modes of the region outside of the top
electrode 135 (between the top electrode edge and the substrate
edge) from coupling to the substrate 105, as evanescent and complex
modes decay exponentially from the excitation edge located at the
edge of the top electrode 135. Further, the temperature
compensating first acoustic impedance layer 107 stabilizes changes
of the sound velocity of the bottom electrode 115, the
piezoelectric layer 125 and the top electrode 135 layers in
response to changes in temperature.
Notably, FIG. 1B depicts a single acoustic resonator 100B. If the
acoustic resonator 100B were to be included in a device with
additional acoustic resonators, for example, in a filter including
5-10 acoustic resonators, the first and second acoustic impedance
layers 107 and 108 of the DBR 106 would need to be electrically
isolated from DBRs of the other acoustic resonators, as would be
apparent to one of ordinary skill in the art. For example, a trench
or other isolating means may be etched off around the DBR 106 down
to the substrate 105.
The bottom electrode 115 may be formed of one or more electrically
conductive materials, such as various metals compatible with
semiconductor processes, including tungsten (W), molybdenum (Mo),
aluminum (Al), platinum (Pt), ruthenium (Ru), niobium (Nb), or
hafnium (Hf), for example. In various configurations, the bottom
electrode 115 may be formed of two or more layers of electrically
conductive materials, which may by the same as or different from
one another. Likewise, the top electrode 135 may be formed of
electrically conductive materials, such as various metals
compatible with semiconductor processes, including tungsten (W),
molybdenum (Mo), aluminum (Al), platinum (Pt), ruthenium (Ru),
niobium (Nb), or hafnium (Hf), for example. In various
configurations, the top electrode 135 may be formed of two or more
layers of electrically conductive materials, which may by the same
as or different from one another. Also, the configuration and/or
the material(s) forming the top electrode 135 may be the same as or
different from the configuration and/or the material(s) forming the
bottom electrode 115.
The substrate 105 may be formed of a material compatible with
semiconductor processes, such as silicon (Si), gallium arsenide
(GaAs), indium phosphide (InP), glass, sapphire, alumina, or the
like, for example. Various illustrative fabrication techniques for
an air cavity in a substrate are described by U.S. Pat. No.
7,345,410 (Mar. 18, 2008), to Grannen et al., which is hereby
incorporated by reference in its entirety. The piezoelectric layer
125 may be formed of any piezoelectric material compatible with
semiconductor processes, such as aluminum nitride (AlN), zinc oxide
(ZnO), or zirconate titanate (PZT), for example.
The first planarization layer 120 may be formed of borosilicate
glass (BSG), for example. The first planarization layer 120 is not
strictly required for the functioning of acoustic resonator 100B,
but its presence can confer various benefits. For instance, the
presence of the first planarization layer 120 tends to improve the
structural stability of acoustic resonator 100B, may improve the
quality of growth of subsequent layers, and may allow bottom
electrode 115 to be formed without its edges extending beyond the
cavity 110. Further examples of potential benefits of planarization
are presented in U.S. Patent App. Pub. No. 2013/0106534 to Burak et
al., which is hereby incorporated by reference in its entirety.
Referring again to FIG. 1B, the acoustic resonator 100B further
comprises a collar 140 disposed on the second planarization layer
130 and the top electrode 135, and a frame 145 disposed in a bottom
portion of the top electrode 135. Although not shown, a passivation
layer may be present on top of the top electrode 135 with thickness
sufficient to insulate all layers of the acoustic stack from the
environment, including protection from moisture, corrosives,
contaminants, debris and the like. The collar 140 may be formed of
a dielectric material of predetermined thickness and width that
substantially surrounds the main membrane region. The dielectric
material may be silicon dioxide (SiO.sub.2), silicon nitride (SiN),
silicon carbide (SiC), aluminum nitride (AlN), zinc oxide (ZnO),
aluminum oxide (Al.sub.2O.sub.3), diamond, diamond like carbon
(DLC), or lead zirconium titanate (PZT), for example. The frame 145
may be formed of one or more conductive or dielectric materials,
such as copper (Cu), molybdenum (Mo), aluminum (Al), tungsten (W),
iridium (Ir), borosilicate glass (BSG), carbon-doped silicon oxide
(CDO), silicon carbide (SiC), silicon nitride (SiN), silicon
dioxide (SiO.sub.2), aluminum oxide (Al.sub.2O.sub.3), aluminum
nitride (AlN), zinc oxide (ZnO), lead zirconium titanate (PZT),
diamond or diamond-like carbon (DLC), for example.
The second planarization layer 130 may be formed of borosilicate
glass (BSG), for example. Notably, the use of a high acoustic
impedance material in the second planarization layer 130, tends to
produce a vertical modal energy distribution across the acoustic
stack in the region of the collar 140 that matches more closely a
vertical modal energy distribution across the acoustic stack in the
active region. This allows a closer match between a vertical
distribution of the modal energy distribution of electrically
excited piston mode in the active region and a vertical modal
energy distribution of the evanescent thickness extensional (eTE1)
mode in the region of the collar 140 at frequencies around the
series resonance frequency Fs of the acoustic resonator 100B. The
eTE1 mode may then decay exponentially in the direction away from
the collar/membrane interface without coupling to other propagating
modes supported by the acoustic resonator 100B structure. This in
turn may result in overall reduced scattering loss in the collar
region and may produce significant improvements in parallel
resistance Rp and quality factor Q. Moreover, use of higher
acoustic impedance materials in the collar 140 and the passivation
layer may also contribute to improved performance for similar
reasons.
Of course, other materials may be incorporated into the above and
other features of acoustic resonator 100B without departing from
the scope of the present teachings.
A double-headed arrow 152 indicates a main membrane region, or
active region, of the acoustic resonator 100B, and dotted vertical
lines indicate a boundary of the main membrane region 152. This
boundary coincides with the edge of the top electrode 135, except
on connecting side 101, where the top electrode 135 extends beyond
the boundary of the main membrane region 152. Double-headed arrows
154 and 156 indicate respective collar and frame regions of
acoustic resonator 100B, and corresponding dotted vertical lines
indicate boundaries of these regions. When viewed from a top angle,
such as that of FIG. 1A, the above regions and their boundaries may
have an apodized shape. As illustrated in FIG. 1B, the collar 140
has an inner edge that is substantially aligned with the boundary
of the main membrane region 152, and the frame 145 has an outer
edge that is substantially aligned with the same boundary.
In the example of FIG. 1B, the main membrane region 152 does not
include the full extent of overlap between bottom and top
electrodes 115 and 135 and piezoelectric layer 125, because the
illustrated right side of top electrode 135 is a connecting edge
and it is not intended to modify the characteristic electrical
impedance at an operating frequency range of the acoustic resonator
100B in any significant way. However, an overlap between the bottom
electrode 115, the piezoelectric layer 125, the top electrode 135
and the substrate 105 in the top electrode connecting edge, usually
referred to as dead-FBAR, may cause significant acoustic energy
loss since piston mode is electrically excited all the way to the
outer perimeter of the cavity 110 in that region, where it may
couple to propagating modes supported by the substrate 105 region.
The presence of the collar 140 in that region may minimize that
unwanted energy loss by mass-loading the top-electrode connecting
edge, which in turn significantly lowers the amplitude of
electrically excited piston mode at an outer edge of the cavity
110.
During typical operation of acoustic resonator 100B, as a part of a
ladder filter, for instance, an input electrical signal may be
applied to an input terminal of the bottom electrode 115 and the
top electrode 135 may be connected to the output terminal. The
input electrical signal may include a time-varying voltage that
causes vibration in the main membrane region. This vibration in
turn produces an output electrical signal at an output terminal of
the top electrode 135. The input and output terminals may be
connected to bottom and top electrodes 115 and 135 via connection
edges that extend away from the main membrane region 152 as shown
in FIG. 1B. For example, from a top view, these connection edges
may be seen to extend outside of an apodized pentagon shape, such
as that illustrated in FIG. 1A. The input and output terminals of
acoustic resonator 100B may be connected to appropriate terminals
of other acoustic resonators forming the ladder filter, for
instance.
The electrically excited piston mode is terminated at the edge of
top electrode 135. This structural discontinuity at the edge of top
electrode 135 presents a significant discontinuity in cutoff
frequencies between the main membrane region 152 and peripheral
regions, and it causes excitation of lateral modes in both the main
membrane and peripheral regions to facilitate continuity of
appropriate particle velocity and stress components at the
interface between these regions. This can lead to undesirable
scattering of acoustic energy from the piston mode and the
resulting degradation of electrical response of acoustic resonator
100B. Collar 140, however, provides mass loading which lowers the
cutoff frequency outside the main membrane region 152, producing a
more laterally uniform cutoff frequency profile across acoustic
resonator 100B. Similarly, frame 145 suppresses electrically
excited piston mode in the frame region, and it reflects and
otherwise resonantly (exponentially) suppresses propagating
(evanescent and complex) eigenmodes in lateral directions, with
both effects simultaneously improving operation of acoustic
resonator 100B. In other words, performance improvement of acoustic
resonator 100B is facilitated by at least one of a cutoff frequency
mismatch and an acoustic impedance mismatch between the frame
region and other portions of the main membrane region 152 which is
produced by frame 145.
Meanwhile, as discussed above, the DBR 106 generally mitigates
acoustic losses in the vertical direction (y-dimension in the
coordinate system depicted in FIG. 1B) of the acoustic resonator
100B, and the temperature compensating first acoustic impedance
layer 107 stabilizes response by offsetting at least a portion of a
negative temperature coefficient of the piezoelectric layer 125,
the bottom electrode 115 and the top electrode 135. The principle
of operation of the DBR 106 relies on the fact that, due to
destructive interference of an incident acoustic wave, its total
amplitude decays exponentially in the direction of propagation
through the acoustic stack (in this case away from the interface
between bottom electrode 115 and first acoustic impedance layer
107). In general, such beneficial exponential decay of wave
amplitude is only possible if the thicknesses of the first and
second acoustic impedance layers 107 and 108 (and any additional
acoustic impedance layers) comprising DBR 106 are equal to or close
to equal to an odd multiple of the quarter wavelength of an
incident acoustic wave. At the bottom of the DBR stack (in this
case at the interface between second acoustic impedance layer 108
and the substrate 105), the wave amplitude is small, thus yielding
negligible radiation of acoustic energy into the substrate 105. In
other words, the acoustic energy incident upon the DBR 106 is being
reflected back with only small transmission of acoustic energy into
the substrate 105. Notably, the beneficial reflectivity properties
of the DBR 106 are in general possible for a limited range of
frequencies, a specific polarization and a limited range of
propagation angles of an incident wave. In practical cases when the
range of frequencies is given by a bandwidth of a filter and
multiple eigenmodes are being excited in the active region, the
optimal thicknesses of the various acoustic impedance layers are
found numerically and experimentally.
Also as mentioned above, the use of two acoustic impedance layers
(e.g., the first and second acoustic impedance layers 107, 108) is
merely illustrative, and the DBR 106 may comprise more than two
acoustic impedance layers, or a single acoustic impedance layer.
The number of acoustic impedance layers provided for the DBR 106 is
determined by a tradeoff between expected reflection performance
(the more layers the better) and cost and processing issues (the
fewer layers the cheaper and more straightforward mirror growth and
post-processing). Furthermore, depending on the acoustic impedance
of the substrate 105, the second acoustic impedance layer 108 may
be foregone, with the first acoustic impedance layer 107 being
disposed directly over the substrate 105 and having its thickness
increased to be half-wavelength (rather than a quarter-wavelength).
The amount of acoustic isolation of the excited eigenmodes provided
by the DBR 106 also depends on the contrast between the acoustic
impedances of the adjacent acoustic impedance layers, with a
greater amount of contrast creating better acoustic reflection of
the eigenmodes with dominant vertical polarization component, as
discussed above.
In an embodiment, the cavity 110 may be formed before the DBR 106,
and filled with sacrificial material, which is subsequently
released after formation of the DBR 106 and the additional layers
of the acoustic resonator 100B. The acoustic impedance layers of
the DBR 106 are provided over the substrate 105 using materials
deposited by known methods. For example, the second acoustic
impedance layer 108 may be formed over the substrate 105, and the
first acoustic impedance layer 107 is formed over the second
acoustic impedance layer 108. Alternatively, the first acoustic
impedance layer 107 may be formed over the substrate 105 directly.
Still alternatively, additional acoustic impedance layers (not
shown) may be provided between the second acoustic impedance layer
108 and the first acoustic impedance layer 107. In all embodiments,
however, the first acoustic impedance layer 107, which has
comparatively low acoustic impedance, is provided beneath the
bottom electrode 115. The layers of the DBR 106 can be fabricated
using various known methods, an example of which is described in
U.S. Pat. No. 7,358,831 (Apr. 15, 2008) to Larson, III, et al., the
disclosure of which is hereby incorporated by reference in its
entirety.
In general, the main membrane region 152 of FBAR 100B is defined by
the presence of air (essentially zero acoustic impedance material)
at both top and bottom boundaries. Therefore vertical stress
components are zero at these boundaries. Similarly, through proper
selection of materials in the DBR 106, the first acoustic impedance
layer 107 may have very low acoustic impedance compared to the
bottom electrode 115, which may also lead to a lowered vertical
stress at the boundary between the bottom electrode 115 and the
first acoustic impedance layer 107. Such a lowered stress condition
is however only possible when thickness of the first acoustic
impedance layer 107 is reasonably close to an odd multiple of the
quarter wavelength of the modes (e.g., in this case electrically
driven piston mode and eTE1 eigenmode) for which the DBR 106 is
being designed. Adding more acoustic impedance layers to the DBR
106 further lowers the vertical stress at the interface between the
bottom electrode 115 and the first acoustic impedance layer 107,
thus allowing for closer approximation of an ideal zero-stress
condition.
However, as mentioned above, while lower vertical stress for
electrically driven piston mode and eTE1 eigenmode is realized by
the selection of the thickness of the first acoustic impedance
layer 107, for other modes which are excited either electrically or
mechanically (by modal coupling at the lateral edges of the
membrane) that may not necessarily be the case and leakage of these
modes through the DBR 106 may be actually enhanced (leading to
lesser than expected energy confinement). For instance, presence of
relatively thick first acoustic impedance layer 107 with low
acoustic impedance generally lowers the cutoff frequency of the
second order thickness shear mode TS2, which in turn increases the
shear component in the eTE1 mode at the parallel resonance
frequency Fp resulting in weaker coupling of eTE1 modes supported
by the collar 140 on either side of the cavity 110 edge. That
weaker coupling causes stronger excitation of propagating modes and
increased radiative loss, as described above in relation to collar
operating principles. In other words, proximity of TS2 resonance to
TE1 resonance in the DBR 106 region may increase lateral leakage of
acoustic energy. To address that problem, a thinner than quarter
wavelength first acoustic impedance layer 107 may be used, which in
turn may adversely reduce overall reflectivity of DBR 106 in
vertical direction and temperature compensating properties of DBR
106. The proper balance between these two leakage mechanisms and
temperature compensation is usually determined by numerical
simulations and experiments.
In general, the depth of the cavity 110 is determined by the etch
properties of the sacrificial material and by possible downward
bowing of the released membrane (i.e., layers of the acoustic
resonator 100B disposed over the cavity 110) in the case of
residual compressive stress in the layers of the membrane being
present. Usually deeper cavities are more beneficial from the
membrane release process point of view, but they also yield
somewhat more difficult initial etch process. If the above
mentioned features of the release process require deeper cavities,
one can increase the depth of the cavity 110 by continued etching
of the substrate 105 until required distance between the DBR 106
and the bottom of the cavity 110 is obtained.
The first and second acoustic impedance layers 107 and 108 have
thicknesses in the range of approximately 1000 .ANG. to
approximately 50000 .ANG., respectively, depending on the material
used and the frequency operating range of the filter. As mentioned
above, the total thickness of all acoustic impedance layers
comprising the DBR 106 is substantially equal to one
quarter-wavelength of the fundamental mode in the selected material
and excited at the selected operational frequency (e.g., series
resonance frequency). For example, if the first acoustic impedance
layer 107 comprises TEOS for operation at about 800 MHz (series
resonance frequency), the first acoustic impedance layer 107 has a
thickness of approximately 2.6 .mu.m. In this example, second
acoustic impedance layer 108 may comprise SiN, having a thickness
of approximately 3.2 .mu.m for operation at about 800 MHz. Notably,
the thickness of all acoustic impedance layers of the DBR 106 can
be selected to be odd-multiple (e.g., 5) quarter-wavelengths of the
fundamental acoustic resonator eigenmode in the selected material
(e.g., if one quarter-wavelength layer is too thin for practical
processing).
Referring to FIG. 1C, the acoustic resonator 100C is similar to the
acoustic resonator 100B, except for formation of the frame 145.
That is, unlike acoustic resonator 100B, in which the frame 145 is
a composite frame (integrally formed within a corresponding
electrode to provide planar top surfaces), the frame 145 in top
electrode 135' of the acoustic resonator 100C is an add-on frame.
An add-on frame results in a substantially non-planar top surface
profile of the top electrode 135'.
Generally, because only a passivation layer usually would be formed
on the top electrode 135', such non-planar profiles of the top
electrode 135' would not have any significant impact on structural
robustness of the acoustic resonator 100C. On the other hand,
frames 150 and 150' in acoustic resonators 100D and 100F, discussed
below, would be composite frames if included in acoustic resonator
100C, resulting in substantially planar top surface profiles of the
electrodes 115. Such substantially planar top surfaces would be
preferable in the bottom electrode 115 of acoustic resonator 100C
in order to form a high quality, void-free piezoelectric layer 125
and top electrode 135'. Some additional general tradeoffs of
different frame configurations are described, for instance, in the
above cited U.S. patent application Ser. No. 13/663,449. Of course,
the structure of the add-on frame 145 may be applied to frames
included in the acoustic resonators 100D through 300D, without
departing from the scope of the present teachings. In addition,
other frame configurations (add-on and composite) may be
incorporated, such as additional frame configurations disclosed by
U.S. patent application Ser. No. 13/781,491, filed Feb. 28, 2013,
for example, without departing from the scope of the present
teachings.
Otherwise, the acoustic resonator 100C includes substrate 105,
cavity 110, DBR 106 disposed over the substrate 105 and the cavity
110, bottom electrode 115 disposed on the DBR 106, and first
planarization layer 120 is disposed on the DBR 106 adjacent to the
bottom electrode 115. Piezoelectric layer 125 is disposed on the
bottom electrode 115 and the first planarization layer 120, and the
top electrode 135 and second planarization layer 130 are disposed
on the piezoelectric layer 125. The DBR 106 includes first and
second acoustic impedance layers 107 and 108, where the first
acoustic impedance layer 107 is formed of a material having
relatively low acoustic impedance and the second acoustic impedance
layer 108 is formed of a material having relatively high acoustic
impedance, although additional acoustic impedance layers may be
included. Also, the first acoustic impedance layer 107 is
configured to act as a temperature compensating layer, in that the
low acoustic impedance material of which it is formed has a
positive temperature coefficient that offsets the negative
temperature coefficients of at least the bottom electrode 115, the
piezoelectric layer 125 and/or the top electrode 135.
FIGS. 1D, 1E and 1F depict additional variations of the acoustic
resonator 100B. In particular, in FIG. 1D, acoustic resonator 100D
is substantially the same as acoustic resonator 100B, except that
frame 145 is omitted and frame 150 is located at a bottom portion
of bottom electrode 115. In FIG. 1E, acoustic resonator 100E is
substantially the same as acoustic resonator 100B, except that
frame 150 is provided at a bottom portion of bottom electrode 115,
in addition to frame 145 in the top electrode 135. In FIG. 1F,
acoustic resonator 100F is substantially the same as acoustic
resonator 100B, except that frame 150' is provided at a top portion
of bottom electrode 115, in addition to frame 145 in the top
electrode 135. Of course, each of the acoustic resonators 100C
through 100F include the DBR 106 with first and second acoustic
impedance layers 107 and 108, where the first acoustic impedance
layer 107 is formed of a material having relatively low acoustic
impedance and the second acoustic impedance layer 108 is formed of
a material having relatively high acoustic impedance, although
additional acoustic impedance layers may be included. The first
acoustic impedance layer 107 is configured also to act as a
temperature compensating layer, in that the low acoustic impedance
material of which it is formed has a positive temperature
coefficient that offsets the negative temperature coefficients of
at least the bottom electrode 115, the piezoelectric layer 125
and/or the top electrode 135.
The frames 150 and 150' in acoustic resonators 100D through 100F
provide benefits similar to frame 145 of acoustic resonator 100B,
although their performance and manufacturing processes will vary
somewhat due to the different locations of the frames. Some general
tradeoffs of different frame configurations are described, for
instance, in the above cited U.S. patent application Ser. No.
13/660,941.
Generally, the temperature compensating first acoustic impedance
layer 107 in acoustic resonators 100B through 100F will tend to
decrease the electromechanical coupling coefficient Kt.sup.2 of the
corresponding acoustic resonator device. In order to compensate,
the piezoelectric layer 125 may be formed of materials with
intrinsically higher piezoelectric coupling coefficient (e.g., ZnO
instead of AlN). Also, in various embodiments, the piezoelectric
layer 125 may be "doped" with one or more rare earth elements, such
as scandium (Sc), yttrium (Y), lanthanum (La), or erbium (Er), for
example, to increase the piezoelectric coupling coefficient
e.sub.33 in the piezoelectric layer 125, thereby off-setting at
least a portion of the degradation of the electromechanical
coupling coefficient Kt.sup.2 of the acoustic resonator caused by
the temperature compensating first acoustic impedance layer 107.
Examples of doping piezoelectric layers with one or more rare earth
elements for improving electromechanical coupling coefficient
Kt.sup.2 are provided by U.S. patent application Ser. No.
13/662,425 (filed Oct. 27, 2012), to Bradley et al., and U.S.
patent application Ser. No. 13/662,460 (filed Oct. 27, 2012), to
Grannen et al., which are hereby incorporated by reference in their
entireties. Of course, doping piezoelectric layers with one or more
rare earth elements may be applied to any of various embodiments,
including the embodiments described with reference to FIGS. 1B
through 3D.
FIGS. 2A-2D are cross-sectional diagrams illustrating acoustic
resonators, according to representative embodiments, including
various arrangements of collars and frames.
FIGS. 2A through 2D are cross-sectional views of acoustic
resonators 200A through 200D, respectively, according to other
representative embodiments. The acoustic resonators 200A through
200D are similar to acoustic resonators 100B through 100F,
respectively, except that collar 140 is omitted and a collar 240 is
instead formed between bottom electrode 115 and piezoelectric layer
125. The collar 240 provides benefits similar to the collar 140 of
acoustic resonators 100B through 100F, although its performance and
manufacture vary somewhat due to the different location of the
collar 240.
Referring to FIGS. 2A through 2D, each of acoustic resonators 200A
through 200D, which may be an FBAR, for example, includes substrate
105, cavity 110, DBR 106 disposed over the substrate 105 and the
cavity 110, bottom electrode 115, 215 disposed on the DBR 106, and
first planarization layer 120 is disposed on the DBR 106 adjacent
to the bottom electrode 115, 215. Piezoelectric layer 125 is
disposed on the bottom electrode 115, 215 and the first
planarization layer 120, and the top electrode 135, 235 and second
planarization layer 130 are disposed on the piezoelectric layer
125. The DBR 106 includes first and second acoustic impedance
layers 107 and 108, where the first acoustic impedance layer 107 is
formed of a material having relatively low acoustic impedance and
the second acoustic impedance layer 108 is formed of a material
having relatively high acoustic impedance, although additional
acoustic impedance layers may be included. Also, the first acoustic
impedance layer 107 is configured to act as a temperature
compensating layer, in that the low acoustic impedance material of
which it is formed has a positive temperature coefficient that
offsets the negative temperature coefficients of at least the
bottom electrode 115, 215, the piezoelectric layer 125 and/or the
top electrode 135, 235.
Note that for illustrative purposes, the frame 245 formed in the
top electrode 235 and frames 250 and 250' formed in the bottom
electrode 215 of the acoustic resonators 200A through 200D are
constructed differently than the frame 145 of in the top electrode
135 and the frames 150 and 150' formed in the bottom electrode 115
of the acoustic resonators 100B through 100F, respectively,
although the functionality of the frames 245, 250 and 250' is
substantially the same as discussed above. In particular, the top
electrode 235 in FIGS. 2A, 2C and 2D is a composite electrode
comprising two different metal materials to provide integrated
lateral features (frames 245). Likewise, the bottom electrodes 215,
215' in FIGS. 2B, 2C and 2D are also composite electrodes
comprising two different metal materials to provide integrated
lateral features (frames 250, 250').
Generally, the frame 245 comprises an inside electrode layer formed
on the piezoelectric layer 125 and an outside electrode layer
formed on the inside electrode layer. The outside electrode layer
is formed of a first material and the inside electrode layer is
formed of the first material and a second material, where the first
material effectively extends from the outside electrode layer
through the second material of the inside electrode layer to
provide the frame 245 (in a bottom portion of the top electrode
235). The second material may have higher (lower) sound velocity
than the first material in order to form a low (high) velocity
frame. For example, for low velocity frame the second material may
be formed of molybdenum (Mo) or aluminum (Al) and the first
material may be tungsten (W), although other materials may be
incorporated without departing from the scope of the present
teachings. The frame 250 comprises an inside electrode layer formed
beneath the piezoelectric layer 125 and an outside electrode layer
formed beneath the inside electrode layer. The inside electrode
layer is formed of the first material and the outside electrode
layer is formed of the first and second materials, where the first
material effectively extends from the inside electrode layer
through the second material of the outside electrode layer to
provide the frame 250 (in a bottom portion of the bottom electrode
215). Similarly, the frame 250' comprises an inside electrode layer
formed beneath the piezoelectric layer 125 and an outside electrode
layer formed beneath the inside electrode layer. The outside
electrode layer is formed of the first material and the inside
electrode layer is formed of the first and second materials, where
the first material effectively extends from the outside electrode
layer through the second material of the inside electrode layer to
provide the frame 250' (in a top portion of the bottom electrode
215').
The frames may be realized by other types and locations of
integrated lateral features formed by composite electrodes, without
departing from the scope of the present teachings. Examples of
composite electrodes with integrated lateral features are provided
by U.S. patent application Ser. No. 13/660,941, filed Oct. 25,
2012, which is hereby incorporated by reference in its entirety.
Also, some general tradeoffs of different frame configurations are
described, for instance, in the above cited U.S. patent application
Ser. No. 13/663,449. Of course, the structures of the frames 245,
250 and 250' may be applied to the acoustic resonators 100B through
100E, discussed above, and the structures of the frames 145, 150
and 150' may be applied to the acoustic resonators 200A through
200D, without departing from the scope of the present
teachings.
In alternative embodiments, the various features of the acoustic
resonators 100B through 200D may be provided in various
combinations that include either a collar (of various types or
locations) or one or more frames (of various types or locations),
but not both collars and frames, without departing from the scope
of the present teachings. For example, FIGS. 3A through 3D are
cross-sectional views of acoustic resonators 300A through 300D,
respectively, according to other representative embodiments, which
include collars or frames, along with other illustrative features
discussed above.
Referring to FIG. 3A, the acoustic resonator 300A is a
representative FBAR that is similar to acoustic resonator 100B,
except that it includes only the collar 140 in addition to the DBR
106 with temperature compensating first acoustic impedance layer
107 (with no frame). Similarly, referring to FIG. 3B, the acoustic
resonator 300B is a representative FBAR that is similar to acoustic
resonator 200A, except that it includes only the collar 240 in
addition to the DBR 106 with temperature compensating first
acoustic impedance layer 107 (with no frame). Referring to FIG. 3C,
the acoustic resonator 300C is a representative FBAR that is
similar to acoustic resonator 100B, except that it includes only
the frame 145 in addition to the DBR 106 with temperature
compensating first acoustic impedance layer 107 (with no collar).
Similarly, referring to FIG. 3D, the acoustic resonator 300D is a
representative FBAR that is similar to acoustic resonator 100D,
except that it includes only the frame 150 in addition to the DBR
106 with temperature compensating first acoustic impedance layer
107 (with no collar). Additional examples similar to acoustic
resonators 300C and 300D are described in U.S. patent application
Ser. No. 13/767,754 to Burak et al., mentioned above, which is
hereby incorporated by reference in its entirety.
Of course, these are only examples of features. Other features and
other combinations of features may be incorporated without
departing from the scope of the present teachings. For example, the
acoustic resonators 300C and/or 300D alternatively may include only
frame 150', both frames 145 and 150, or both frames 145 and 150'.
Also, various different arrangements and/or types of frames (e.g.,
frames 145, 150, 150', 245, 250, 250') (composite or add-on) and
collars (e.g., collars 140, 240), as discussed above, may be
incorporated, without departing from the scope of the present
teachings. The DBRs and corresponding temperature compensating
features, the frames, and the collars provide benefits similar to
those discussed above, although performance and manufacture varies
somewhat due to different locations and combinations.
FIG. 4 is a graph illustrating the quality factor (Q-factor) and
parallel resistance Rp of the acoustic resonators of FIGS. 3A and
1B, as compared to an acoustic resonator having only an acoustic
reflector (e.g., DBR 106) with a temperature compensating acoustic
impedance layer (e.g., first acoustic impedance layer 107 formed of
material having a positive temperature coefficient). That is, FIG.
4 illustrates simulated Q-factor and parallel resistance Rp
comparisons of acoustic resonators with a temperature compensating
acoustic impedance layer in an acoustic mirror (e.g., temperature
compensating first acoustic impedance layer 107), with and without
a collar (e.g., collar 140), and with and without a collar and an
integrated frame (e.g., frame 145), as are shown in FIGS. 3A and
1B, for example. The purpose of theses graphs is to illustrate
changes in performance of the pass-band of the acoustic resonators
that occur as a consequence of adding the collar 140 and the frame
145 to the acoustic reflector with a temperature compensating
acoustic impedance layer. Referring to FIG. 4, Q-factor is
represented by a y-axis on the left side, and Rp is represented by
a y-axis on the right side. The values of the Q-factor and Rp are
shown as functions of input signal frequencies on an x-axis.
In the example of FIG. 4, the dimensions of the acoustic resonator
(e.g., acoustic resonator 100B) have been tuned for high Rp. In
particular, bottom electrode 115 is formed of Mo with a thickness
of approximately 3800 .ANG., piezoelectric layer 125 is formed of
AlN with a thickness of approximately 9300 .ANG., top electrode 135
is formed of Mo with a thickness of approximately 3250 .ANG., and a
passivation layer is formed of AlN (over the top electrode 135)
with a thickness of approximately 2000 .ANG.. With regard to
alignments, an outer edge of the bottom electrode 115 extends
approximately 5.5 .mu.m outside the cavity 110, and an outer edge
of the top electrode 135 extends approximately 5.0 .mu.m within the
cavity 110.
Further, in the example, the DBR 106 includes first acoustic
impedance layer 107 formed of tetra-ethyl-ortho-silicate (TEOS),
with a thickness of approximately 7500 .ANG., and second acoustic
impedance layer 108 formed of W with a thickness of approximately
4500 .ANG., optimized for best parallel resistance Rp for the
particular scenarios, discussed below. Thus, the first acoustic
impedance layer 107 may operate at a series resonance frequency of
about 2.7 GHz and the second acoustic impedance layer 108 may
operate at a series resonance frequency of about 2.9 GHz. With
regard to collars and frames (when included), the collar 140 is
formed of SiC at a thickness of approximately 9500 .ANG. (i.e.,
about 4250 .ANG. thicker than the top electrode 135 and the
passivation layer) and a width of approximately 11.5 .mu.m, and the
frame 145 is a composite frame formed of approximately 350 .ANG.
thick and approximately 3.75 .mu.m wide Al embedded in the Mo at
the bottom of the top electrode 135.
FIG. 4 depicts three configurations, each of which is indicated by
a corresponding Q-factor curve and Rp value curve. In particular,
curves 410Q and 410R respectively illustrate the Q-factors and the
Rp values of a bare resonator (FBAR) with the temperature
compensating DBR 106 having temperature compensating first acoustic
impedance layer 107. This design generally corresponds to acoustic
resonator 300A shown in FIG. 3A, but without the collar 140.
Similarly, curves 420Q and 420R respectively illustrate the
Q-factors and the Rp values of the acoustic resonator with the
temperature compensating DBR 106 and the collar 140 (e.g., acoustic
resonator 300A), and curves 430Q and 430R respectively illustrate
the Q-factors and the Rp values of the acoustic resonator with the
temperature compensating DBR 106, the collar 140, and the frame 145
(e.g., acoustic resonator 100B). More specifically, curves 410R,
420R and 430R illustrate magnitudes of complex-valued electrical
impedance of the acoustic resonator.
At parallel resonance frequency Fp electrical impedance becomes
approximately real-valued and the peak value of electrical
impedance magnitude indicates parallel resistance Rp. A peak value
of the Q-factor occurs for each of the curves 410Q through 430Q at
about 1.925 GHz. This frequency corresponds to the series resonance
frequency Fs of the respective acoustic resonators. Similarly, peak
values of Rp occur for each of the curves 410R through 430R in a
range of about 1.962 GHz to about 1.966 GHz. These frequencies
correspond to the parallel resonance frequency Fp of the respective
acoustic resonators. The bandwidths of these acoustic resonators
correspond to the range of frequencies between their respective
values of series resonance frequency Fs and parallel resonance
frequency Fp.
Referring to FIG. 4, it is apparent that including the combination
of all features (temperature compensating DBR 106, collar 140 and
frame 145) in the acoustic resonator improves performance of the
acoustic resonator. For example, adding the collar 140 (curves 420Q
and 420R) increases parallel resistance Rp by about five times.
Further adding the frame 145 (curves 430Q and 430R) increases the
parallel resistance Rp about another two times (about a ten times
increase in parallel resistance Rp overall). In particular, curve
410R (temperature compensating DBR 106 only) indicates an Rp value
of about 570 Ohms, curve 420R indicates an Rp value of about 2700
Ohms and curve 430R indicates an Rp value of about 5700 Ohms. Also,
at frequencies above the series resonance frequency Fs, the
acoustic resonator has significantly higher Q-factor, indicated by
curves 420Q and 430Q in comparison to curve 410Q. As should be
appreciated by one of ordinary skill in the art, the Q-factor and
Rp values of the acoustic resonator increase without significant
degradation of the bandwidth when compared to the acoustic
resonator with only temperature compensating DBR 106.
Notably, by adding the collar 140 and the frame 145, radiative loss
is substantially eliminated, essentially flattening the Q-factor
values at and above the series resonance frequency Fs, as indicated
by curve 430Q. Increased series Q-factor Qs (decreased series
resistance Rs) is due to lowered scattering at the series resonance
frequency Fs. However, for frequencies below the series resonance
frequency Fs, the Q-factor curve 410Q for a bare resonator with a
temperature compensating DBR 106 becomes very irregular with
distinct peaks and valleys between neighboring frequencies. This
irregular spectral pattern, or so-called "rattles," is caused by
excitation of a propagating thickness extensional mode pTE1-, e.g.,
at the edge of the top electrode 135 for driving frequencies below
series resonance frequency Fs (by analogous mechanism described
above in relation to excitation of eTE1 mode for driving
frequencies above the series resonance frequency Fs). As should be
appreciated by one skilled in the art, for so-called type-II
acoustic stacks (as considered in the present teaching), the
evanescent eTE1 mode above series resonance frequency Fs
corresponds to propagating pTE1-mode below series resonance
frequency Fs. A phase of acoustically excited pTE1-mode may change
very rapidly as the driving frequency changes, which in turn may
produce a rattle in the Q-factor spectrum, as seen in curve 410Q,
for example.
The presence of the collar 140 in acoustic resonator 300A allows
the piston mode in the main membrane region to couple predominantly
to eTE1 mode in the collar region of the collar 140 rather than to
pTE1-mode of main membrane region, provided that the designed
collar's cutoff frequency is slightly below the driving frequency.
As a result, rattles below series resonance frequency Fs are
suppressed, as indicated by circle 425 for curve 420Q. Such
simultaneous suppression of rattles below series resonance
frequency Fs and increase of Q-factor above series resonance
frequency Fs is generally very beneficial for filter performance,
for example. As mentioned above, addition of a low velocity
composite frame 145 as in the acoustic resonator 100B, for example,
enables an increase in parallel resistance Rp by approximately
three times as compared to the acoustic resonator 300A without the
frame. The increase of parallel resistance Rp is however
accompanied by increased rattles in the acoustic resonator 100B at
frequencies below the series resonance frequency Fs, as indicated
by circle 435 for curve 430Q. The increase of rattles' amplitude is
caused by the fact that in the low velocity frame region of the
frame 145, the piston mode is driven in opposite phase to the
piston mode in the main membrane region 152, causing enhanced
pTE1-mode excitation at the main region/frame region interface.
Thus, numerical and experimental optimization of the respective
widths and thicknesses of the frame 145 and collar 140 may be
performed to simultaneously decrease rattles below series resonance
frequency Fs and increase Q-factor and parallel resistance Rp above
resonance frequency Fs to the desired level.
In the above-described embodiments, DBRs and corresponding
temperature compensating features, collars and frames can generally
be formed using conventional processing techniques, with examples
including various forms of deposition, sputtering, etching,
polishing, and so on. Moreover, the described embodiments and
related methods of fabrication can be modified in various ways as
will be apparent to those skilled in the art.
In accordance with various embodiments, a low acoustic impedance
layer functioning as a temperature compensating layer in a DBR
formed over a cavity, combined with one or more frames and/or
collars, create weakly confined structures that minimize parasitic
scattering of electrically excited piston mode, and therefore
create acoustically lossless acoustic resonator, such as an FBAR or
SMR. Generally, the collar couples piston mode and eTE1 mode of the
main membrane region to evanescent mode of the collar region, the
DBR effectively eliminates dead-FBAR and minimizes coupling of the
eTE1 mode of the collar to the resonator substrate while providing
at least partial temperature compensation of the frequency
response, and the frame (e.g., composite frame) suppresses
excitation of propagating modes.
While example embodiments are disclosed herein, one of ordinary
skill in the art appreciates that many variations that are in
accordance with the present teachings are possible and remain
within the scope of the appended claims. For instance, as indicated
above, the location, dimensions, and materials of a collar and/or
frames can be variously altered. In addition, other features can be
added and/or removed to further improve various performance
characteristics of the described devices. These and other
variations would become clear to one of ordinary skill in the art
after inspection of the specification, drawings and claims herein.
The invention therefore is not to be restricted except within the
spirit and scope of the appended claims.
* * * * *